1. Field of the Invention
This invention relates to a wave abatement device and, more particularly, to an abatement device which dissipates energy in waves and stops the chronic erosion process that occurs along shorefronts.
The ability of the shoreline or shorefront to withstand the force of the waves, and therefore to also withstand erosion, is important particularly where there are homes, businesses and other structures present on the property or the property has an aesthetic appearance that needs to be maintained. Normally, such shorefront property is at a premium both in availability and cost. Erosion of shorefronts, as widely known, occurs because of storms and hurricanes, but, as less widely known, also occurs because of the daily, continuous effect of waves striking the shorefront. Specifically, the more turbulent or shorter period or higher frequency waves, with frequency being a function of the height and period of the wave, the less time the sand which is carried in the wave has to drain out onto the shorefront and also the less time sand already on the beach has to dissipate the force or energy of the wave and thereby prevent being carried away from the beach with the backwash of the wave. Concerning the latter, the water absorption capability of the beach, which is the important dissipator of wave energy, is decreased so that there results an increased backwash energy that moves the sand seaward.
Should one intend to protect a man-made structure, such as a marina, in a body of water, it is necessary to dissipate the energy of the wave before the wave strikes the structure. If the energy of the wave is not dissipated, the continued pounding of the structure by the waves shall cause the structure to eventually crumble. Further, if the energy of the waves is not dissipated then the shorefront may erode and the marina may be damaged.
A longshore movement of sand, known as littoral drift, has been found to be advantageous since it moves sand towards the shoreline thereby replenishing the beach or shorefront. Littoral drift has its greatest effect in replenishing the shorefront during calm water surges to the beach, which normally occur during late spring to mid-summer, since such surges provide more than adequate time to dissipate the energy of the wave so that there is virtually little, if any, backwash energy. Accordingly, the sand builds up on the beach or shorefront.
2. Description of the Prior Art
The two common ways to control erosion and dissipate wave energy, along shorefronts have been heretofore by non-structural and structural stabilization efforts. Non-structural efforts, which have been preferable, involve either the planting of beach grass which acts to capture sand, or the placing of large quantities of sand on the beach, or a combination of both. The planting of beach grass is still normally insufficient to retain enough sand to avoid erosion, and is not possible when a man-made structure, such as an oil rig and the like is to be protected from the waves. The replacement of sand or the transporting of sand to a beach or a marina is very expensive as well as time consuming. Accordingly, no non-structural stabilization effort has proven totally satisfactory to abate erosion.
The variety of structural stabilization efforts include the building of walls, such as jetties and groins, perpendicular to the beach (or marina) to trap sand and hence build up the beach, and the building of walls parallel to the beach, such as longard tubes, seawalls and bulkheads to deflect the waves, and therefore the wave forces, from striking the beach. Each of these structural stabilization efforts has proven to have serious drawbacks which include short design life, the potential for causing subsequent greater damage to the beach, and their erosion effect on downdrift beach or shorefront areas.
For example, as discussed in Chapter 9 of the CRC Handbook of Coastal Processes and Erosion (1983), walls built perpendicular to the beach, namely jetties, have been found to block the littoral drift. By doing so, the sand is prevented from reaching the downdrift side so that while sand is captured at the desired location, neighboring downdrift locations erode even faster. Likewise, a groin, which is a rib built approximately perpendicular to the beach or shoreline, traps the littoral drift so that the local updrift side of the shoreline is built up. Unfortunately, as with jetties, groins build up the updrift side by preventing sand from reaching the downdrift side so that the neighboring downdrift side erodes even faster. The building of several groins in succession increases the build-up of the updrift side, while simultaneously increasing the rate of erosion of the downdrift side. Further, jetties, groins, and other such breakwater means are designed to be struck by waves. This action causes their bases to erode rather quickly, and therefore these structures shall readily require replacement. Still further, these type structures are very costly to construct.
Walls constructed parallel to the shoreline, such as longard tubes, seawalls and bulkheads or breakwaters, deflect waves, instead of absorbing the waves. Accordingly, these structures also erode rather quickly, thus causing their materials to wash-up on the shorefront. Also, each of these structures has been found to produce other deleterious effects. For example, Coastal Engineering, 8 (1984) pp.199-217, in an article titled: Performance Documentation of the Longard Tube at Del Mar, Calif. 1980-1983 by R. E. Flick and B. W. Waldorf, states that the study monitoring the Longard Tube installation at Del Mar beach from December 1980 until early 1983 principally concluded that the tube as configured was not substantial enough barrier to effectively prevent beach erosion during severe storm events. Specifically, the Longard Tubes, which are low-cost sand-filled plastic tube devices positioned parallel to the shoreline, were found inadequate to prevent large waves from overtopping the tube thereby producing localized sand scour on the shoreward side as the water returned seaward. As the scouring continued, the water returning seaward poured over the tube and began to also scour channels on the tube's seaward side. This process continued to perpetuate until the tubes were rendered totally ineffective. Also, a seawall, as stated in CRC Critical Reviews in Environmental Control, September 1976, pp. 259-296, in an article titled: Beach Erosion: Causes, Processes, and Remedial Measures by Dean, results in greater erosion at the location of the seawall toe than if the seawall were not present. Further, the seawall, as with any structure deflecting the waves, has its base scoured out so that the base of the seawall will eventually erode.
Recognizing the ineffectiveness of the above known processes to abate erosion, others have directed their attention toward decreasing the energy in waves before the waves strike the shorefront. For example, U.S. Pat. No. 3,197,963 to Frederiksen, which issued on Aug. 3, 1965, is directed to an apparatus which serves as a temporary breakwater by attempting to attenuate the wave action and to decrease the relative height of the peak with respect to the bottom of the trough of the wave. Specifically, the apparatus includes a plurality of enclosed, elongated hollow bodies oriented in a single row and in side-by-side relation so that each of the elongated bodies extends in the direction of wave travel. Each body has a bladder-like upper portion and a rigid bottom. The enclosed body is provided with a quantity of gaseous fluid, such as air, preferably up to approximately 40% to 60% of its volume capacity. When the hollow tube is anchored submerged in the water, it operates as follows. As the crest of the wave imposes upon the top exterior of the partially inflated hollow body, the bladder upper portion collapses under the crest to thereby collapse a portion of the chamber. However, due to the influence of the water at the trough, the air pocket formed under the trough travels along the hollow body. This action causes a collapse of the wave crest and the filling of the trough by the generation of another wave. Clearly, this is a temporary measure which would necessarily not last for a long period of time due to the natural dissipation of the gas, as well as the probability that it would not have any significant effect on high frequency waves.
An apparent attempt to combine the deflection and wave absorption or abatement concepts to abate erosion is U.S. Pat. No. 1,353,001 to Uriarte, which issued on Sept. 14, 1920. It is directed to a breakwater or the like. Specifically, the breakwater includes a solid masonry or other suitable structure comprising a series of substantially horizontal perforated diaphragms one above the other, and vertical cutwaters in front of the diaphragms. Waves striking the structure are divided into a number of vertical sections, and then are subdivided vertically by the diaphragms. The problem with this structure is that it is costly, and possibly may erode by the water striking it.